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United States Patent |
5,344,792
|
Sandhu
,   et al.
|
September 6, 1994
|
Pulsed plasma enhanced CVD of metal silicide conductive films such as
TiSi.sub.2
Abstract
In semiconductor manufacture, a pulse plasma enhanced chemical vapor
deposition (PPECVD) method is provided for depositing a conductive film of
low resistivity on a substrate. The PPECVD method is especially suited to
the deposition of metal silicides such as TiSi.sub.x on a silicon
substrate during contact metallization. The PPECVD method can be carried
out in a vacuum reaction chamber of a cold wall CVD reactor. A metal
precursor deposition gas such as TiCl.sub.4 is reacted with a silicon
source gas such as SiH.sub.4 at a deposition temperature of about
500.degree. C. For generating a pulsed plasma, an rf power supply is
coupled to the reaction chamber and to a pulse generator.
Inventors:
|
Sandhu; Gurtej S. (Boise, ID);
Doan; Trung T. (Boise, ID)
|
Assignee:
|
Micron Technology, Inc. (Boise, ID)
|
Appl. No.:
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026525 |
Filed:
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March 4, 1993 |
Current U.S. Class: |
438/660; 117/92; 117/939; 257/E21.165; 313/231.31; 438/680; 438/681; 438/683 |
Intern'l Class: |
H01L 021/44 |
Field of Search: |
437/200,174
313/231.31
156/657,613
|
References Cited
U.S. Patent Documents
4608271 | Aug., 1986 | Heiber et al. | 437/200.
|
4877641 | Oct., 1989 | Dory | 427/38.
|
4883686 | Nov., 1989 | Doehler et al. | 427/38.
|
4935661 | Jun., 1990 | Heimecke et al. | 313/231.
|
4977106 | Dec., 1990 | Smith | 437/192.
|
5084417 | Jan., 1992 | Joshi et al. | 437/192.
|
5160408 | Nov., 1992 | Long | 156/657.
|
Foreign Patent Documents |
1149965 | Jun., 1989 | JP.
| |
Other References
Kang et al., "Dependence of oxygen redistribution on titanium film
thickness during titanium silicide formation by rapid thermal annealing",
J. Vac. Sci. Technol A7(6), Nov./Dec. 1989, pp. 3246-3250.
Rosler et al., "Plasma-enhanced CVD of titanium silicide", J. Vac. Sci.
Technol B2(4), Oct.-Dec. 1984, pp. 733-737.
Lee et al., "Plasma Enhanced Chemical Vapor Deposition of Blanket
TiSi.sub.2 on Oxide Patterned Wafers", J. Electrochem, Soc., vol. 139, No.
4, Apr. 1992, pp. 1159-1165.
Kwong et al, SPIE, vol. 1189, Rapid isothermal processing (1989), pp.
109-120.
Ho et al "Formation of self-aligned TiSi.sub.2 for VLSI contacts and
Interconnects", J. Vac. Sci. Tech, A5(4), Ju.-Aug. (1987), pp. 1396-1401.
|
Primary Examiner: Breneman; R. Bruce
Assistant Examiner: Paladugu; Ramamohan Rao
Attorney, Agent or Firm: Gratton; Stephen A.
Claims
What is claimed is:
1. In semiconductor manufacture a CVD method for depositing a metal
silicide conductive film on a substrate comprising:
placing the substrate in a CVD reaction chamber;
maintaining the reaction chamber under vacuum pressure;
introducing a metal precursor deposition gas and a silicon source gas into
the reaction chamber for reaction to deposit the metal silicide conductive
film on the substrate wherein said metal precursor gas is selected from
the group consisting of an organic or inorganic metal source and said
silicon source gas is selected from the group consisting of an organic or
inorganic silicon source;
generating a plasma from the gases within the reaction chamber by
introducing a plasma forming energy into the reaction chamber;
pulsing the plasma energy to shift the dynamic equilibrium of the plasma
for reaction mechanics for depositing the metal silicide conductive film
and diffusing reactant by products away from the substrate;
adjusting a pulse power, a pulse duration and a gas chemistry of the metal
precursor deposition gas and silicon source gas during deposition to
improve deposition reaction mechanics and form the metal silicide film
with an as deposited resistivity; and then
annealing the metal silicide conductive film using a rapid thermal anneal
to lower the as deposited resistivity.
2. The CVD method as recited in claim 1 and wherein the plasma is generated
using a power source selected from the group consisting of modulated rf
power, microwave power, or DC power.
3. The CVD method as recited in claim 2 and wherein the reaction chamber
comprises a cold wall CVD reactor.
4. The CVD method as recited in claim 3 and wherein the metal silicide
conductive film is TiSi.sub.2 having a resistivity after rapid thermal
annealing of about 20 .mu..OMEGA.-cm.
5. In semiconductor manufacture, a pulse plasma enhanced chemical vapor
deposition (PPECVD) method for depositing a metal silicide conductive film
on a substrate, comprising:
placing the substrate in a CVD reaction chamber of a cold wall reactor said
substrate including a material selected from the class of materials
consisting of silicon, polysilicon or SiO.sub.2 ;
maintaining the reaction chamber under a vacuum pressure;
introducing a metal precursor deposition gas and a silicon source gas into
the reaction chamber for reaction with the metal silicide conductive film
on the substrate wherein said metal precursor deposition gas is selected
from the group consisting of a metal halide, a metal dialkyamide,
(TiCl.sub.4), (TDEAT), (TDMAT), (TZAT), or (Tris (2, 2' bipyridine)
titanium) and said silicon source gas is selected from the group
consisting of (SiH.sub.4), (Si.sub.2 H.sub.6), (SiBr.sub.4), (SiF.sub.4),
(SiH.sub.2 Cl.sub.2), (HMDS), (DMDCS), (TMCS), (DMDMOS), or (BDMADMS);
generating a plasma from the gases within the reaction chamber using an rf
power source coupled to the reaction chamber;
pulsing the rf power to form a pulsed plasma;
adjusting the rf power and a gas chemistry of the metal precursor
deposition gas and silicon source gas during deposition of the metal
silicide conductive film to improve deposition reaction mechanics and form
the metal silicide conductive film with an as deposited resistivity; and
then
annealing the metal silicide conductive film using a rapid thermal anneal
to lower the as deposited resistivity.
6. The PPECVD method as recited in claim 4 and wherein the power source is
an rf pulse generator pulsed at a frequency of between 50 Hz-10,000 Hz.
7. The PPECVD method as recited in claim 6 and wherein a deposition
temperature is between about 200.degree.-700.degree. C., and a silicon
source gas flow is between about 5-40 sccm.
8. The PPECVD method as recited in claim 7 and wherein the pulses are with
a duty cycle in range of 15-80%.
9. In semiconductor manufacture, a PPECVD method for depositing a
TiSi.sub.2 film on a substrate comprising:
placing the substrate in a CVD reaction chamber of a cold wall reactor said
substrate including a material selected from the class consisting of
silicon, polysilicon or SiO.sub.2 ;
maintaining the reaction chamber under a vacuum pressure;
introducing a titanium precursor deposition gas selected from the group
consisting of, a metal dialkyamide, (TiCl.sub.4), (TDMAT), (TDEAT),
(TZAT), or (Tris (2, 2' bipyridine) titanium) into the reaction chamber
along with a reactant silicon source gas selected from the group
consisting of (SiH.sub.4), (Si.sub.2 H.sub.6), (SiBr.sub.4), (SiF.sub.4),
SiH.sub.2 Cl.sub.2), (HMDS), (DMDCS), (TMCS), (DMDMOS), or (BDMADMS) for
reaction with the precursor deposition gas for depositing the TiSi.sub.2
film on the substrate;
generating a plasma from the gases within the reaction chamber by
introducing a plasma forming energy into the reaction chamber;
pulsing the plasma energy to shift a dynamic equilibrium of the plasma for
the reaction mechanics for depositing the TiSi.sub.2 film; and
adjusting the plasma energy, a pulse duration and a gas chemistry to form
the TiSi.sub.2 film with an as deposited resistivity; and then
annealing the TiSi.sub.2 film using a rapid thermal anneal to lower the as
deposited resistivity.
10. The PPECVD method as recited in claim 9 and wherein the metal precursor
deposition gas is (TiCl.sub.4) and the silicon source gas is (SiH.sub.4).
11. The PPECVD method as recited in claim 9 and wherein the plasma is
generated with an rf power supply that is pulsed using a pulse generator.
12. The PPECVD method as recited in claim 9 and wherein the TiSi.sub.2 film
is used in a contact metallization process to form low resistivity
contacts.
13. The PPECVD method as recited in claim 12 and wherein the deposition
temperature is from 200.degree.-700.degree. C. and a flow rate of the
silicon source gas is between about 5-40 sccm.
14. The PPECVD method as recited in claim 13 and wherein the deposited
TiSi.sub.2 has a bulk resistivity of about 20 .mu..OMEGA.-cm.
Description
FIELD OF THE INVENTION
This invention relates to the deposition of films in semiconductor
manufacture and particularly to a pulse plasma enhanced chemical vapor
deposition method for depositing conductive films. The method of the
invention is especially suited to the deposition of TiSi.sub.x films in a
semiconductor contact metallization process.
BACKGROUND OF THE INVENTION
Microchip fabrication involves the formation of integrated circuits (ICs)
on a semiconducting substrate. A large number of semiconductor devices are
typically constructed on a monolithic substrate of a single crystal
silicon material. The semiconductor devices are formed by various
processes such as doping and patterning the substrate and by depositing
various conducting and insulating layers on the substrate.
The continued miniaturization of integrated circuits has brought about an
increasing need to reduce resistivities in the source-drain-gate regions
and of the contact metallurgy to these regions. Recently, much effort has
been focused on the use of different conducting materials and metal
silicides to form such contacts.
One conducting material that is used in such applications is titanium
nitride (TiN). In the past, titanium nitride films have been used in
semiconductor manufacture for local interconnects and as contacts to
semiconductor devices. Recently, titanium nitride films have been used in
advanced metallization technology for manufacturing ultra large scale
integrated circuits (ULSI).
In these applications, titanium nitride may be used as a diffusion barrier
against junction spiking for aluminum contacts to silicon. In addition,
titanium nitride may be used as a glue layer between tungsten (W) and
inter-metal dielectrics in a semiconductor structure. Titanium nitride may
also be used to preserve the integrity of junctions in a semiconductor
structure from worm hole effects during the chemical vapor deposition
(CVD) of tungsten.
Improved processes have recently been developed for depositing conductive
films such as titanium nitride, particularly for contact metallization. As
an example, different methods of chemical vapor deposition (CVD) and
plasma enhanced chemical vapor deposition (PECVD) of titanium nitride have
recently been developed.
In general, single phase titanium nitride films deposited onto a silicon
substrate with a CVD process have a high contact resistance. For this
reason, during metallization and contact formation using titanium nitride,
a separate conductive layer of titanium is typically deposited on the
silicon substrate. During the deposition process, the titanium layer is
thermally reacted with silicon on the surface of the substrate to form a
layer of TiSi.sub.x, such as titanium silicide (TiSi.sub.2). This process
is sometimes referred to as silicidation. The layer of TiSi.sub.x can be
used to form contacts having low contact resistance and good barrier
properties.
This technology however, is not entirely suitable to the manufacture of
ultra large scale integrated circuits (ULSI). Such high density circuits
are formed with semiconductor devices having scaled down features. The
contact junctions formed in the silicon substrate are thus relatively
small and shallow in depth. Since the silicon in the junction region is
consumed during the silicidation process in proportion to the thickness of
the silicide that is formed, the shallow junction structure may be
adversely affected during silicidation process. Specifically, it is
difficult to make low resistance, reliable and thermally stable titanium
silicide contacts to shallow junctions. Another problem associated with
this technology is that reliable techniques for the deposition of highly
conformal TiSi.sub.x films have not heretofore been developed.
In order to address these problems, various low pressure chemical vapor
deposition (LPCVD) processes for depositing a layer of TiSi.sub.x on
silicon have been proposed. As an example, LPCVD processes have been
studied using a titanium precursor such as titanium tetrachloride
(TiCl.sub.4) and a source of silicon such as silane (SiH.sub.4) to deposit
TiSi.sub.x film.
A problem with such processes is that reaction temperatures in excess of
730.degree. C. are required. These temperatures are above the melting
point of some of the materials used to form the semiconductor structure.
Aluminum, for instance, melts at a temperature of about 600.degree. C.
Another shortcoming of these prior art processes is that a seed layer of
polysilicon may be required in order to deposit any TiSi.sub.x on a
silicon substrate. Moreover, the films deposited in accordance with these
methods have poor adhesion and rough surfaces.
Plasma enhanced chemical vapor deposition (PECVD) has also been utilized in
the deposition of TiSi.sub.x films. Although a PECVD process can be
performed at lower temperatures than the LPCVD processes previously
described with PECVD, the bulk resistivity of the deposited films is
relatively high (>250 .mu..OMEGA.-cm). Furthermore, films deposited using
PECVD are relatively rough.
As is apparent from the foregoing, there is a need in the semiconductor art
for improved methods for depositing conductive films and particularly
titanium silicide (TiSi.sub.x) films on silicon for contact metallization.
Accordingly, it is an object of the present invention to provide an
improved method for depositing conductive films such as TiSi.sub.x on a
substrate utilizing pulsed-plasma enhanced chemical vapor deposition
(PPECVD). It is a further object of the present invention to provide a
pulsed plasma enhanced chemical vapor deposition (PPECVD) method in which
high quality conductive films, and particularly TiSi.sub.x films can be
conformally deposited on a substrate. It is yet another object of the
present invention to provide a pulsed plasma enhanced chemical vapor
deposition process for depositing TiSi.sub.x and other conductive films,
on a substrate in which a deposited film is characterized by a smooth
surface and a low bulk resistivity. Finally, it is an object of the
present invention to provide a pulsed-plasma enhanced chemical vapor
deposition (PPECVD) method for depositing TiSi.sub.x and other conductive
films on a substrate that is suitable for large scale semiconductor
manufacture and particularly semiconductor contact metallization for ULSI
circuits.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method of depositing conductive
films on a substrate for semiconductor manufacture is provided. The method
utilizes pulse plasma enhanced chemical vapor deposition (PPECVD) to
deposit high quality conformal conductive films on a substrate (e.g.
silicon, polysilicon, SiO.sub.2). The PPECVD is carried out in a cold wall
reactor at vacuum pressures and relatively low temperatures (e.g.
500.degree. C.). During PPECVD, a metal precursor deposition gas is
reacted with a silicon source deposition gas. A pulsed plasma is generated
using a high frequency rf power source modulated with a pulse generator.
The deposited films are characterized by a low resistivity, fine grain
structure and a smooth surface. Following deposition, a rapid thermal
anneal (RTA) step can be used to reduce the resistivity of the deposited
films even lower.
By pulsing the plasma power "on" and "off" or between "high" and "low"
levels, the reaction mechanics of the deposition process are improved. In
particular, during high pulse power the reaction mechanics for depositing
the reactant species onto the substrate are optimized. During a low or off
pulse power, the diffusion of by products from the substrate is optimized.
The pulsed plasma thus leads to a shift in the dynamic equilibrium of the
plasma and changes the average density of various intermediate ion species
present in the reaction chamber. This change in the reaction kinetics can
be controlled for depositing films with desirable characteristics.
In an illustrative embodiment of the invention, TiSi.sub.x is deposited on
a silicon substrate during contact metallization for forming low
resistivity contacts. For depositing TiSi.sub.x on silicon, a titanium
precursor deposition gas, such as titanium tetrachloride (TiCl.sub.4), is
reacted with a silicon source deposition gas, such as silane (SiH.sub.4),
in a cold wall reactor. The rf power, duty cycle and modulation frequency
of the rf power source can be varied to achieve desired film
characteristics.
Other objects, advantages and capabilities of the present invention will
become more apparent as the description proceeds.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a cold wall reactor for carrying out the
method of the invention;
FIG. 2 is a graph showing the sheet resistivity of TiSi.sub.x films
deposited in accordance with the invention, with the sheet resistivity
shown as a function of duty cycle at an rf power of 50 W and a modulation
frequency of 2500 Hz;
FIG. 3 is a graph showing the sheet resistance of TiSi.sub.x films
deposited in accordance with the invention, with the sheet resistivity
shown as a function of silane flow at a constant TiCl.sub.4 flow rate of 5
sccm, a pressure of 800 mT and an rf power of 50 W;
FIG. 4 is a graph showing the bulk resistivity of TiSi.sub.x films
deposited in accordance with the invention, with the bulk resistivity
shown as a function of the deposition temperature; and
FIG. 5 is a graph showing the sheet resistance of TiSi.sub.x films
deposited with a conventional prior art plasma deposition process as
compared to the sheet resistance of TiSi.sub.x films deposited in
accordance with the method of the invention as a function of the power of
an rf source.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The method of the invention, broadly stated, comprises the pulsed plasma
enhanced chemical vapor deposition (PPECVD) of a conductive film on a
substrate. The PPECVD process of the invention is especially suited for
depositing a metal silicide such as TiSi.sub.x on a silicon, polysilicon,
or SiO.sub.2 substrate.
For depositing a metal silicide on a substrate, the method of the invention
includes the steps of:
1. Placing the substrate in a CVD reaction chamber.
2. Maintaining the reaction chamber under a vacuum pressure.
3. Introducing a metal precursor deposition gas into the reaction chamber
along with a silicon source gas for reaction with the metal precursor gas
for depositing a conductive film on the substrate.
4. Generating a plasma from the gases within the reaction chamber by
introducing a plasma forming energy into the reaction chamber.
5. Pulsing the plasma energy to shift the dynamic equilibrium of the plasma
for optimizing the reaction mechanics for depositing the conductive film.
6. Adjusting the parameters of the deposition process to form a film with
predetermined characteristics.
In an illustrative embodiment, the PPECVD method of the invention is
performed in a cold wall CVD reactor. An rf power source is coupled to the
reaction chamber to provide a plasma energy. For pulsing the plasma
between an "on" and "off" or a "high" and "low" level, the rf power is
pulsed or modulated using a pulse generator.
Other methods of generating an energy pulse for pulsing the plasma may also
be utilized. These alternate pulse generating methods may include a
pressure pulse, a heat pulse, a laser pulse, microwave modulation, and
others.
With an rf power source, process parameters such as rf power, duty cycle
and modulation frequency have a significant effect on the characteristics
of the deposited film. In addition, process parameters such as deposition
temperature, pressure and gas flow rates also have a significant effect on
the characteristics of the deposited film. In accordance with the
invention, each of these parameters can be adjusted to produce a film
having desired characteristics.
The PPECVD method of the invention is especially suited to the formation of
metal silicide contacts and interlayer interconnects in semiconductor
structures. Conductive materials other than metal silicides, however, such
as titanium, tungsten, molybdenum, tantalum and metal nitrides such as
titanium nitride, tungsten nitride and tantalum nitride may also be
deposited.
For depositing a metal silicide, a suitable metal precursor gas for the
deposition reaction can be formed from an organic or inorganic metal
source. For the deposition of TiSi.sub.x, suitable titanium sources
include TiCl.sub.4 ; [Ti(N(R).sub.2).sub.4 ] where R.dbd.CH.sub.3 (TDMAT)
where R.dbd.C.sub.2 H.sub.5 TDEAT; Cp.sub.2 Ti (N3).sub.2 where
Cp.dbd.C.sub.5 H.sub.5 group (biscyclopentadienyl titanium diazide);
Cp.sub.2 Ti Cl.sub.2 where Cp.dbd.C.sub.5 H.sub.5 group (biscyclo
pentadienyl titanium dichloride); and Tris (2, 2' bipyridine) titanium.
Other suitable titanium sources are listed in the example to follow. Other
conductive layers such as refractory metal silicides may be deposited
utilizing a suitable metal halide such as pentachlorides, hexachlorides,
pentafluorides, and hexafluorides.
For depositing a metal silicide film, a silicon source gas is also required
for reaction with the metal precursor deposition gas. Either an organic or
inorganic silicon source gas may be utilized. Suitable silicon source
gases include silane (SiH.sub.4), disilane (Si.sub.2 H.sub.6), silicon
tetrabromide (SiBr.sub.4), silicon tetrafluoride (SiF.sub.4) and
dichlorosilane (SiH.sub.2 Cl.sub.2). Other suitable organosilicon
compounds include hexamethyldisilazane (HMDS), dimethyldichlorosilane
(DMDCS), trimethylchlorsilane (TMCS), dimethyldimethoxsilane (DMDMOS) and
bis(dimethylamino)-dimethysilane (BDMADMS). However, any functionally
equivalent organosilicone compound would be within the scope and spirit of
the present invention.
Films can be deposited in accordance with the PPECVD method of the
invention with an extremely low resistivity. Following deposition, a rapid
thermal annealing (RTA) step can be used to further lower the resistivity.
Films deposited in accordance with the PPECVD method of the invention are
further characterized by a smooth surface and a fine dense grain
structure.
A suitable cold wall CVD reactor 14 for carrying out the method of the
invention is shown in FIG. 1. The cold wall CVD reactor 14 includes a
reaction chamber 16 wherein a silicon wafer 18 may be located on a
graphite or steel boat 20. The graphite boat 20 is heated to a desired
temperature by halogen lamps 21. The cold wall CVD reactor 14 may also
include a premix chamber 22 wherein the gases are mixed prior to being
directed through a shower head 24 into the reaction chamber 16. The cold
wall CVD system may also include pressure control means in the form of a
pressure sensor 26, a pressure switch 28, an air operated vacuum valve 30,
and pressure control valve 32. In addition, reactant gases given off by
the chemical reaction are drawn by a roots blower 34 into a particulate
filter 36 and to the atmosphere.
A supply of carrier gases 38 are coupled through suitable valving to the
premix chamber 22. The carrier gases may include Argon, Nitrogen, and
Helium and other suitable inert gases. A supply of the metal precursor
deposition gas 40 is also coupled to the premix chamber through suitable
valving. A silicon source gas 42, such as a silane (SiH.sub.4), is also
coupled to the premix chamber 22 using suitable valving.
With this arrangement the inert carrier gases, the metal precursor
deposition gas, and the silicon source gas can be combined in the premix
chamber 22. The temperatures and flow rates of the gases can be controlled
to achieve the desired reaction and film quality.
For generating a plasma, a pair of rf electrodes 44, 46 is located within
the reaction chamber 16. A grounded electrode 44 is connected to the
chamber walls. A positive electrode 46 is located within the reaction
chamber 16. The rf electrodes 44, 46 are connected to a power supply 48.
The power supply 48 is a modulated rf power source which creates a radio
frequency (rf) field through the electrodes 44, 46 and in the reaction
chamber 16. With such a cold wall CVD reactor 14 the power may be on the
order of 25 W to 500 W. Alternately, other power sources such as microwave
power or DC power may be utilized in place of rf power.
The rf field energizes the gas mixture to a plasma state. In general, the
plasma comprises mobile, positively and negatively charged particles.
These particles interact because of the attraction or repulsion resulting
from the electric field surrounding each charged particle, i.e. Coulomb
forces. Plasma particle species include neutral atoms, electrons and ions.
Typically, the density of opposite charges in a gaseous plasma is equal
and thus the plasma is electrically neutral. The reacting gases are
ionized and dissociated by electron impact. These ions then combine on the
surface of the substrate as the deposition compound.
In accordance with the method of the invention, the rf power for forming
the plasma is modulated using a pulse generator. The pulse generator
functions to change the amplitude of the power "on" and "off" or from
"high" to "low". This cycles the plasma state between an "on" state and an
"off" state and between a "high" to "low" plasma density state.
In addition, the pulse generator may be adjusted for changing the pulse
amplitude, pulse frequency, and duty cycle of the pulses. The frequency of
the pulse generation may be from 50 Hz to 10,000 Hz. In addition, the duty
cycle of the pulses has a significant effect on the characteristics of the
deposited film. Such a duty cycle can be defined as a percentage of the
pulse time period during which the pulse is "on".
It is theorized that at an on, or high power phase, of a pulse cycle, the
deposition of the reactant species onto the substrate proceeds at an
optimal rate. On the other hand, during the off or low power phase of a
pulse cycle, the diffusion of reaction by-products away from the substrate
proceeds at an optimal rate. Stated differently, during the off phase,
reaction by-products are pumped away so that the deposition process can
proceed more efficiently. The use of the pulsed plasma thus leads to a
shift in the dynamic equilibrium of the plasma. This changes the average
density of various intermediate ion species present in the reaction
chamber. By controlling the process parameters (i.e. power, pulse,
temperature, gas streams, chemistries), the reaction kinetics can be
controlled for depositing films with desirable characteristics (i.e. low
resistivity, smooth surface, dense grain structure).
As an example, the reaction chemistry can be controlled such that a
constant low concentration of silicon ions is maintained in the reaction
chamber. With a low silicon concentration, the Si content of a deposited
silicide can be kept low. This produces films with a low resistivity.
The flow rates of the metal precursor deposition gas, the silicon source
gas and the carrier gases also have a significant effect on the deposited
film. By way of example, the flow rate of the carrier gases (Ar, N.sub.2
He) may be as great as five to ten times the flow rate of the silicon
source gas. The flow rate of the silicon source gas in turn may range from
a percentage of the flow rate of the metal precursor deposition gas (i.e.
silicon gas flow<precursor gas flow) up to 100 times greater than the flow
ratio of the metal precursor deposition gas. For a CVD reactor 14 as
previously described, gas flow rates in the range of 1-50 sccm are
representative.
The temperature at which the substrate is maintained also has a significant
effect on the characteristics of the deposited film. For a CVD reactor 14
as described above, process temperatures in the range of
200.degree.-700.degree. C. can be utilized. Process pressures for such a
CVD reactor 14 may be in the range of 1 m Torr-5 Torr.
EXAMPLE--DEPOSITION OF TiSix
TiSi.sub.x conductive films were deposited in accordance with the method of
the invention on a silicon oxide substrate in a semiconductor
metallization process. For depositing TiSi.sub.x, a precursor deposition
gas was formed from titanium tetrachloride (TiCl.sub.4). Argon was used as
a carrier gas to bubble through the TiCl.sub.4 contained in a quartz
ampule at 50.degree. C.
Alternately, for depositing TiSi.sub.x, the precursor deposition gas may be
selected from the group consisting of titanium tetrachloride (TiCl.sub.4),
[Ti (N(R).sub.2).sub.4 ], where R.dbd.CH.sub.3 (TDMAT) where R.dbd.C.sub.2
H.sub.5 TDEAT; Cp.sub.2 Ti(N3).sub.2 where Cp.dbd.C.sub.5 H.sub.5 group
(biscyclopentadienyl titanium diazide); Cp.sub.2 TiCl.sub.2 where
Cp.dbd.C.sub.5 H.sub.5 group (biscyclo pentadienyl titanium dichloride);
Tris (2, 2' bipyridine) titanium; or a metal dialkylamide such as
Ti(NMe.sub.2).sub.4 ; Ti(NEt.sub.2).sub.4 ; Ti(N-n-Pr.sub.2).sub.4 ; or Ti
(N-n-Bu.sub.2).sub.4.
A silicon source gas for depositing the TiSi.sub.x was silane (SiH.sub.4).
Alternately, a silicon source gas may be selected from an organic or
inorganic source as previously identified.
Depositions were performed in a cold wall single wafer reactor over a wide
range of temperatures, pressures, gas flow rates, and rf power. The films
were deposited on silicon and silicon dioxide substrates. The deposited
TiSi.sub.x films were compared to films deposited by conventional
non-pulsed PECVD.
Pulsing the plasma results in a stringent control of the plasma chemistry.
During an off or low power state of the plasma, reaction by-products can
be pumped away. This helps to maintain the thermodynamic equilibrium of
the reaction species in the plasma. For the case of TiCl.sub.4 and
SiH.sub.4 plasma, SiH.sub.4 is more easily ionized (as compared to
TiCl.sub.4). Accordingly, with a non-pulsed deposition its reactive
concentration will keep building as the deposition progresses. In general,
with a non-pulsed PECVD deposition TiSi.sub.x films rich in silicon are
deposited. Silicon rich films have a high resistivity.
With a pulsed plasma enhanced chemical vapor deposition (PPECVD), on the
other hand, a constant low concentration of reactive silane can be
maintained in the reaction chamber. This is because during the off state,
ions become neutral through recombination. By controlling the reaction
chemistry in this manner, stoichiometric low resistivity films can be
deposited. Furthermore, with PPECVD the inclusion of impurities can be
reduced.
Table 1 shows the resistivity of TiSi.sub.x films deposited using a
non-pulsed (PECVD) as compared to TiSi.sub.x films deposited with similar
process parameters using pulsed plasma enhanced chemical vapor deposition
(PPECVD) in accordance with the invention.
TABLE 1
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For average plasma power = 30 Watts, 0.6
Torr pressure, substrate temperature = 450.degree. C., 10
sccm silane, and 200 sccm Ar flow, followed by RTP
anneal in N.sub.2 at 800.degree. C. for 30 seconds.
PLASMA CONDITION Rb (micro-ohms cm)
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Non pulsed 300
Pulsed at 100 Hz 55
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In general, films deposited with pulsed plasma were highly reflective as
compared to films deposited with a non-pulsed deposition process. In
addition, pulse deposited films had a very low stress of 3.5 E9
dynes/cm.sup.2. Moreover, a pulsed plasma produces a film with better step
coverage. This is believed to be caused by an improved mean free path of
the reactive species on the wafer surface.
For the case of PPECVD films at a fixed power and modulation frequency, the
sheet resistance increased with the duty cycle as shown in FIG. 2. One
could surmise that an increase in the duty cycle in effect, leads to
higher average rf power which could result in higher resistivity. However,
when a lower duty cycle at higher power was used to match the average rf
power, lower resistivity films were deposited.
The deposition rate and resistivity of the films varied with gas flow rates
and the ratio of TiCl.sub.4 and SiH.sub.4 flow rates. FIG. 3 shows the
variation of sheet resistivity of the "as deposited" and "after RTP" films
with silane flow. In general, resistivity of the films was lowered after
annealing in N.sub.2 ambient at 800.degree. C. for 20 seconds. FIG. 3
shows that the degree to which film resistivity is lowered after RTA
varies with the partial pressure of silane. Films deposited at 25 sccm
silane flow show the lowest sheet. This film had a sheet resistivity
equivalent to a bulk resistivity of 20 .mu..OMEGA.-cm after RTA as shown
in FIG. 3. The bulk resistivity of the as deposited films decreased at
higher deposition temperatures as shown in FIG. 4. It is believed that at
higher deposition temperatures, the microstructure and crystal structure
of the film changes over to a low resistivity phase.
Films deposited using pulsed PECVD showed a dramatic improvement in surface
topography. In general, the grain density and fineness is much higher with
the PPECVD films. In addition, PPECVD films do not show any surface
irregularities. Film adhesion to a SiO.sub.2 substrate was excellent
except for the case of very low silane flows and rf power.
FIG. 5 compares the sheet resistance as a function of rf power of
TiSi.sub.x films deposited using PPECVD versus films deposited using
conventional PECVD. At the same power, a decrease in resistivity of more
than an order of magnitude is obtained with pulse plasma enhanced CVD.
In conclusion, pulsed plasma enhanced chemical vapor deposition (PPECVD)
can be used to deposit stoichiometric TiSi.sub.x films at a temperature of
500.degree. C. using TiCl.sub.4 as a metal precursor source and SiH4 as a
silane source. As deposited film resistivities decreased at higher
deposition temperatures. Films with a bulk resistivity as low as 20
.mu..OMEGA.-cm after post deposition rapid thermal anneal have been
obtained. Compared to conventional PECVD, films deposited using PPECVD
showed lower resistivity and a smoother surface texture for the same
average rf power. The films also showed good adhesion to the substrate.
Although the invention has been described in terms of a preferred
embodiment for depositing a TiSi.sub.x film, other silicides and
conductive layers can also be deposited in accordance with the invention.
Thus, it is intended that alternate embodiments of the inventive concepts
expressed herein be contained within the scope of the following claims.
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